1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system such as a printer, a copying machine and a facsimile machine to form an image on a sheet (i.e., paper or clear film on which a toner image is formed) with toner.
2. Description of Related Art
FIG. 21 is a distribution graph of electric charges of toner used for an image forming apparatus of an electrophotographic system. As illustrated in FIG. 21, for example, in the case that charge polarity of the toner for forming a toner image is negative, there exists reversal charge toner charged positive within the toner while a large amount of ordinary charge toner charged negative is in the toner.
FIG. 22 is a sectional view which illustrates the configuration of a development device 4. As illustrated in FIG. 22, the toner is often charged due to friction with a developer bearing member 41 of the development device 4. When friction is generated at the toner due to the rotation of the developer bearing member 41, the ordinary charge toner and the reversal charge toner are generated at a specific probability and they are newly generated in the toner when new toner is supplied.
As illustrated in FIG. 22, the development device 4 has a development container 40. A developer amount restriction member 43 is arranged at an opening of the development container 40. A developer bearing member 41 is arranged at the opening surrounded by the development container 40 and the developer amount restriction member 43. One half of the developer bearing member 41 is arranged inside the development container 40 and the other half is arranged outside the development container 40. A magnet 42 is arranged as “magnetic field generation unit” inside the developer bearing member 41. A development bias supply 47 is connected to the developer bearing member 41. The developer bearing member 41 is arranged to be opposite to a photosensitive drum 1.
FIG. 23 is a plane view which illustrates the occurrence of shadowing at the surface of a sheet m. As illustrated in FIG. 23, when the sheet m is moved in the proceeding direction A, the reversal charge toner charged positive is accumulated inside the development device 4. This may lead to the occurrence of imaging failure called shadowing having patterns like a shadow at the front side of the image formed on the sheet m.
FIG. 24 is a graph which illustrates the relation between fog (%) and sheet smoothness (s). There is a case that the abovementioned reversal charge toner causes reversal fog at white parts of the image. It is known that the shadowing gets worse when the reversal fog gets worse. With high sheet smoothness, both the reversal fog and the shadowing are to worsen. This is because, in the case of using a high smoothness sheet (i.e., coated paper having especially high smoothness), closeness of the sheet and the photosensitive drum is enhanced and clearance to avoid the toner from being pressed to the sheet is eliminated. It is considered that the toner is transferred to the sheet in the abovementioned state by pressure due to enhanced tightness between the sheet and the image bearing member.
FIG. 25 is a graph which illustrates the relation between the fog and fog elimination potential difference. Direct current component potential Vdevdc of bias applied to the developer bearing member in the development device is measured while non-image portion potential Vni of the photosensitive drum is measured. The fog elimination potential difference Vb is expressed by an absolute value of potential difference between the non-image portion potential Vni of the photosensitive drum and the direct current component potential Vdevdc of the development bias (see FIG. 3). As illustrated in FIG. 25, the fog is set at the minimum value when the fog elimination potential difference Vb is 150 V. Ordinary fog and reversal fog are included in the fog. Ordinary fog is defined as the fog caused by the toner expected to form an image. Reversal fog is defined as the fog caused by the toner having the opposite charge polarity to the charge polarity of the toner forming an image. The majority of the charge polarity of the toner when charged by friction is the charge polarity of the toner which causes the ordinary fog. The reversal fog gets worse as the fog elimination potential difference Vb becomes larger than 150 V. On the contrary, the ordinary fog gets worse as the fog elimination potential difference Vb becomes smaller than 150 V. In general, the fog elimination potential difference Vb is set so that the fog is to be minimum when changing the fog elimination potential difference Vb.
An invention disclosed, for example, in Japanese Patent Application Laid-open No. 2002-214943 (hereinafter, called Patent Document 1) has been proposed for suppressing such fog. In the image forming apparatus of the invention described in Patent Document 1, the fog toner on the photosensitive drum becomes harder to be transferred to a thick sheet such as a postcard by decreasing contact pressure between an intermediate transfer belt and the photosensitive drum.
However, the following problem still remains with the image forming apparatus described in Patent Document 1. That is, with the image forming apparatus of Patent Document 1, since mechanical parts exist for switching the contact pressure, time loss, namely, productivity reduction of the image forming, is caused. Such productivity reduction of the image forming causes delayed deliveries, sales reduction and profit reduction for printing companies. It is no exaggeration to state that the above is an issue of vital importance.
The present invention provides an image forming apparatus capable of performing high quality image forming by suppressing reversal fog and shadowing while maintaining high productivity without adding a special mechanical part.
To solve the problem, an image forming apparatus includes:                an image bearing member on which a latent image is formed;        a development apparatus which develops the latent image formed on the image bearing member with toner;        a transfer apparatus which transfers the toner image formed on the image bearing member to transfer material;        an input portion to which information for smoothness of the transfer material onto which the toner image is transferred is inputted; and        a controller which is capable of controlling a potential difference between a non-image portion potential of the image bearing member and a direct current component potential of bias applied to the development apparatus at the time of forming the toner image based on the information inputted to the input portion, the controller being capable of controlling to decrease the potential difference in a case that the information inputted to the input portion is equal to or higher than a predetermined smoothness as compared to a case of being lower than the predetermined smoothness.        
Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).